On the night before the Stevens report asserted that Princess Diana was killed by accident rather than murder, the movie director Oliver Stone flew into London for a Guardian interview at the National Film Theatre. It's an amusing coincidence that the director of JFK and Nixon - cinema's laureate of conspiracy theory - should happen to be in town for the publication of Britain's nearest equivalent to the Warren Commission report. But even the twitchiest online plot-spotter will accept that the director's presence was coincidence rather than conspiracy.

Lord Stevens is unlikely to be so lucky. His 900 pages are intended to be the end of the story but, for the community of the institutionally suspicious, they can never be so. Because their central allegation is that the princess was killed by the British establishment, refutation from a man who received a peerage for a lifetime of service to the police will be the equivalent of a press release from Texaco calling global warming a myth.

Mohamed Al Fayed, a father driven mad by grief, has already suggested that the spies blackmailed Lord Stevens to make him perjure himself. So let's look at this as coolly as we can. In examining conspiracy theories, the key questions are about motives and means. For example, few believe that President Kennedy was killed by Lee Harvey Oswald acting alone, because there are so many groups with more plausible motives (Cuba, Asia, the mob, the military) and because one of Oswald's shots seems forensically unlikely to have achieved the effects it did.

Conversely, we can be sure that Nasa really did land a spacecraft on the moon rather than fake it all in a movie studio because, while a motive for deceit can just about be raised, the means - which would have involved ensuring the lifelong silence of numerous key personnel - are beyond belief.

So what, according to the doubters, was the motive of the palace and MI6 for having Diana killed? Because, stupid, she was pregnant with a half-Muslim half-brother to the second in line to the throne.

Stevens, though, skewers this theory with two types of blood. A sample from Diana lacked the procreative hormones; while Rosa Monckton, a friend who holidayed with the princess 10 days before her death, is insistent that Diana was menstruating during that trip, a fact that a close female companion might reasonably know.

Now, of course, our spooks are more than capable of having swapped Diana's tell-tale test-tube for the blood of a virgin Parisian nun, but can we really believe that Rosa Monckton is such a lackey of the British establishment that she would heroically invent this period detail, while also keeping her journalist husband, Dominic Lawson (no relation), from the story of the century?

Bloodied by these inconvenient truths, conspirators will now regroup and argue that the mere future possibility of pregnancy and marriage were enough to put the black spot on Diana. But, logically, wouldn't a Diana marriage into the Fayed family have put her further outside a royal family that had already stripped her of her title, and perhaps also usefully loosened the backing of her core monarchist support?

Any suggestion that the forces of monarchist conservatism might have gained from the death of Charles's ex-wife is weakened by the fact that the public reaction to her death brought modern Britain closer to republicanism than ever before. And the quality - if not the exact quantity - of the outpouring was predictable. Diana conspiracy theories all falter on the paradox of the plotters being clever enough to fix the hit, but too stupid to anticipate the potential consequences of her loss.

Weakened on motive, the Diana murder hypothesis totally collapses on means. Leaving aside the question of Henri Paul's blood-alcohol level (which requires a lot to be taken on trust), let's concentrate on his observable actions. If he was on a secret mission to steer into a pillar, then he must have been a kamikaze agent, a rare phenomenon in western espionage. Realising this, the plot-spotters now make Paul the patsy, merely driving a car that was diverted to its doom by another agent in the mysterious white vehicle, which may have tailgated the princess's limo, or by a roadside spook aiming a flash gun at the driver's eyes.

The difficulty with this is that the calculations of such an assassination are almost as complex as faking a moon landing. How could White Car Man or Kerbside Flasher be certain of diverting the target vehicle so precisely that a passenger in the back would definitely be killed? As it happened, a bodyguard in the front survived the impact (because of an air bag, says Stevens), while a seat-belt might have spared Diana. By any standards, a state murder so dependent on variables would be highly inefficient.

It can be taken as a sign of Lord Stevens' honour that he includes unexplained details that will encourage conspiracy nuts. Henri Paul seems indeed to have been in the pay of French intelligence, and Diana's phone was being tapped by the Yanks.

But, just as conspiracy theories have a weakness for making things too neat, real history has a tendency to include perplexing loose ends. At the risk of being thought part of a global conspiracy of denial, Lord Stevens has decisively shown that the answer to Diana's death lies not in the files of MI6 but in the pamphlets of the Ministry of Transport. Seat belts and air bags save lives; speeding and drink-driving kills.